How to convert radians to degrees while using Math..asin

Question

If

trace(Math.sin((Math.PI/6))); // .499999 Correct

trace(Math.asin((.499999))); // .523597 Correct

but

trace(Math.sin(30*Math.PI/180)); // 0.49999 Correct

trace(Math.asin(.49999*Math.PI/180)); // 0.008726 Wrong

else

trace(Math.asin(.49999*180/Math.PI)); // 30.000 Correct ?

My Math.asin function should use the radian to degrees conversion factor of Math.PI/180 but appears to be 180/Math.PI.

Can anyone explain why this is?

kglad · Accepted Answer

the trig functions are the inverse of the arc trig functions. the trig functions have an argument that's an angle (in radians) and yield a number. the arc functions have a number for an argument and yield an angle (in radians).

so yes, if you want the sin of 30 degrees, use:

Math.sin(30*Math.PI/180); // converted angle to radians

if you want the angle (in degrees) whose sin is .5, use:

Math.asin(.5)*180/Math.PI

and while our brains are perfectly capable of working with radians without degree conversion, when it comes to digital computers, that doesn't work so well. a digital computer isn't go to tell you asin(PI/4) = sqrt(2)/2. and while i've seen the decimal output of that so many times, i know .707...is sqrt(2)/2, i would not recognize the radian equivalent of 35 degrees. (and i have a phd in math.)

kglad · Answer

to convert rad to deg, multiply by 180/Math.PI

Math.asin(.5)*180/Math.PI; // should be 30 (degrees)

i don't know if you have typos in this forum (ie, your parenthesis are misplaced), but some of what you posted (adjusting the argument in asin) is non-sense from a mathematical point of view.

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